The invention relates to a riser guide system for use on a floating offshore platform. The platform comprises a topsides and a substructure having a lower pontoon, and at least one riser extends from a subsea location to the topsides.
The invention also relates to a method for installing a riser and a riser guide system on a floating offshore platform, and methods for removing riser guides on a floating offshore platform.
In offshore hydrocarbon production, hydrocarbons flow from a subterranean formation into a well, and up to the sea bed. From the sea bed the hydrocarbons flow to a platform via risers. Risers can also be used for water or gas injection, in order to maintain the pressure in the reservoir, or for supplying pressurised hydraulic oil and electric signals for energising and controlling subsea equipment which is used in the hydrocarbon production.
In shallow and medium depth waters fixed platforms resting on the sea bed are used. In deep seas a structure resting on the sea bed would be too large, and therefore floating platforms are used. Due to the motion of the sea, a floating platform is almost always moving. The risers may be stiff steel risers, which are prone to overstressing due to the motions of the floating platform. In order to overcome the problem of the moving platforms, flexible risers may be used. Flexible risers are, however, more expensive than stiff risers.
Irrespectively of what type of risers are used, they must to some extent be laterally guided. Typically riser guides will be located at the pontoon. The riser guides may include sliding pads which are located close to or in abutment with the riser, for laterally guiding the riser during the movement of the platform.
WO 00/58598 discloses a riser guide system comprising a framework which is located around the riser and secured to the platform. Rollers, in the illustrated embodiment having a number of four, are located in the framework, close to or in abutment with the riser, for laterally guiding the riser.
Usually riser guides will be installed subsea, maybe at the pontoon 20-30 meter below the sea surface. This installation may be carried out by divers or an ROV (remotely operated vehicle). This can be dangerous and problematic, and it is therefore desirable to find other ways to do this installation, without divers or an ROV.
Riser guides will after some time be worn, and they must therefore be replaced. Divers or an ROV may be used, but again this can be dangerous and problematic, and is desirable to find other ways of replacing the riser guides.
An object of the invention is to provide a riser guide system which allows installing and replacing the riser guides from the topsides. A further object is to provide a method for installing a riser and a riser guide system on a floating offshore platform, in which offshore operations shall be carried out from the topsides. A further object is to provide a method for removing riser guides on a floating offshore platform, which shall be carried out from the topsides. A particular object is that the invention shall be suitable for stiff risers.
The invention thus relates to a riser guide system for use on a floating offshore platform. The platform comprises a topsides and a substructure having a lower pontoon, and at least one riser extends from a subsea location to the topsides. The system comprises:
a permanent guide having the shape of a housing and having a through-going opening for the riser, the permanent guide is secured to the pontoon,
a primary guide having the shape of a housing and having a through-going opening for the riser, the primary guide is located in the permanent guide,
a secondary guide having the shape of a housing and having a through-going opening for the riser, the secondary guide is located in the primary guide, and riser guides located in the secondary guide and facing the riser.
The riser guide system according to the invention may be used both for flexible and stiff risers. The invention is, however, regarded as particularly favourable for stiff risers, which have a greater need for guiding than flexible risers.
When installing a riser and a riser guide system according to the invention on a floating offshore platform, the following steps are carried out:
a) securing a permanent guide having a through-going opening to the pontoon, the permanent guide has the shape of a housing,
b) placing a primary guide having a through-going opening around a lower riser section at the topsides, the primary guide has the shape of a housing,
c) lowering the lower riser section and the primary guide down to and into the permanent guide,
d) connecting another riser section to the lower riser section and interconnecting more riser sections into a riser, and lowering the riser from the topsides until it extends to a subsea location,
e) placing a secondary guide having a through-going opening around the riser at the topsides, the secondary guide has the shape of a housing, the riser guides are located in the secondary guide and face the riser, and
f) lowering the secondary guide down to and into the primary guide.
The securing of the permanent guide to the pontoon is preferably done prior to the offshore installation of the riser. The lowering of the riser, the primary guide and the secondary guide, which are done offshore, can be carried out from the topsides.
The invention provides two methods for removing riser guides which form part of a riser guide system according to the invention.
In the first method the following steps are carried out:
a) lifting the secondary guide up from the primary guide to the topsides, and
b) removing the riser guides from the secondary guide.
In the second method the following steps are carried out:
a) lifting the primary guide including the secondary guide up from the permanent guide to the topsides, and
b) removing the riser guides from the secondary guide.
Both the lift of the primary guide and the lift of the combination of the primary guide and the secondary guide can be carried out from the topsides. A secondary guide with new or repaired riser guides can then be placed around the riser, and lowered in place. A replacement of the riser guides from the topsides has thereby been carried out.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.